There are two main ways of rolling metal slabs, by rolling a slab through a hot rolling process only or by rolling a slab through a hot rolling process and then a cold rolling process.
In each of the hot and cold rolling methods, rolling techniques can be divided into two methods; reverse rolling methods and tandem rolling methods. In the case of hot rolling of carbon steel there is a reversible rough rolling mill and a five to seven stand tandem-type finishing mill. With hot rolling installations dedicated to rolling stainless steel, a rough rolling mill and a reversing finishing mill having furnace coilers on each side, referred to as "Steckel Mills", are the most common.
Rather than being divided into roughing mills and finishing mills, cold rolling mills are divided into two types: tandem mills for large-scale production; and reverse rolling mills for small-scale production.
With reverse rolling, five to nine passes is usual in hot rolling while three to five passes are common in cold rolling. In tandem rolling, five to seven stands is typical in hot rolling while four to six stands is typical in cold rolling in order to obtain a product thickness in one pass.
Because of this, the disparity in the production volumes of the reversing method and the tandem method is quite large.
Further, in Japanese Utility Model Publication No. Sho. 59-30308 disclosing that relating to rolling of steel bars and wire into cylindrical shapes, a thrust mechanism for regulating shifts of the center of cylinders is disclosed that improves precision of cylinder centering using spherical-shaped supports.
In Japanese Laid-open Patent Publication No. Hei. 5-317918, a tilting member is provided at a sliding surface of a roll chock and a housing in order to dramatically reduce the frictional resistance during raising and lowering of roll chocks for both the work rolls and the back-up rolls to improve the rolled material thickness and strip shape.
Currently, there is a large disparity between the production volume of the reversing method and the tandem method with no practical alternative in response to demands for production volumes lying therebetween.
Reverse rolling is carried out using single rolling mills and one would assume if two rolling mills are used then approximately twice the production volume should be possible. The reasons that this has not been achieved are as follows:
In the case of a hot rolling mill the inter-stand distance is approximately 6 meters. The leading end of the rolling material is then sent from one stand to the next stand, the strip walks and bends so the pass center is displaced from the center and difficulty occurs in biting at the next stand. When biting is carried out offset from the center, strip curving and strip walking occurs, and strip threading does not go well, resulting in a poor strip profile and thickness.
In tandem mills each stand carries out rolling in one direction so strip thickness does not change in one stand of the mill and controlling strip walking is therefore not too difficult. On the other hand, the pass direction changes in reverse rolling methods so that the thickness of the strip being rolled becomes thinner with each pass, i.e. the rolling conditions change. The reduction levelling operation therefore has to be carried out to a high degree of skill and precision, and if the most appropriate level control is not carried out, the strip will walk.
As the amount of strip walking of the leading end is approximately proportional to the square of the length of the inter-stand distance, the amount of strip walking with an inter-stand distance of 6 m is quite large. As a result, strip biting at the following stand is not carried out smoothly and when a walking strip is bitten, strip walking is substantially exacerbated because the strip is not inputted to the center of the rolling mill.
Coilers are provided at the front and rear of a reverse cold rolling mill for coiling and uncoiling during rolling. During this time it is usual for the trailing end of the strip to remain wound onto the coiler. The yield can be improved if the ends of the strip are rolled but strip end passing and recoiling of the strip then becomes very time-consuming, so the productivity falls.
To overcome these difficulties, an invention is put forward with the object of making the distance between two stands as short as possible.
The distance between the centers of the stands can be brought down from 6 m to 3.5 m. Even with this distance side guides are still required for guiding the plate. However, this configuration calls for an airtight space in between the two stands so operation and maintenance becomes troublesome.
Particularly, when strip breakage and crimping occurs, the roll housing becomes a hindrance and the extraction of scrap becomes extremely difficult.